(another) vapor barrier in bathroom question

[Wesley M]

Hi All,

I've scoured this forum and others, and there seems to be limited consensus on this question. So, here is my situation--advice appreciated!

We are remodeling a bathroom in a 1948 house. The room is on the second floor (attic above), with one exterior wall. The short wall of the tub is on the exterior face.

When we pulled the wall down to the studs, there was no insulation in the walls. We do have vinyl siding and rigid foam attached to the outside of the original siding. Most of the indoor surface of the exterior board has a thin black coating (not mold) on it.

For the tile installation around the tub, we will be putting poly behind the cement board (overlapping the tub flange).

I have installed unfaced batts in the exterior wall. Should I install a poly barrier around the exterior wall? I cannot get enough access behind the tub to fully install vapor barrier down to the bottom plate behind the tub on the exterior wall.

Is it worth it to put a vapor barrier up on the wall with the batts, or is it better to leave the vapor barrier off to promote drying if there's an issue?

Thanks for reading!

[Rick Cantrell]

Install a moisture barrier on the studs behind the cement board.
I do not know if Poly is the right product to use, I use 15 lb builders paper (tar paper). Do not worry about the area behind the tub.

[John Kogel]

In my climate, PNW, we will always install poly on the interior side of the unfaced fiberglass batts. Use Tuck Tape to seal all edges of the poly. You need to prevent moisture from ever getting into the fiberglass. It will not dry itself out.
At the far end of the tub, it doesn't hurt to have an access panel. From there, you can check on conditions under the tub from time to time. Not that anyone will. :D

[Robert Ernst]

You should have an air barrier on the outside wall of the tub. It can be anything that is not air permeable but if you can't do it now you should try to seal anything around the tub. Most of the time people install plastic wrong and that's why it traps moisture. I also depends on your climate. Either way you would have to install it air tight and looks like at this point it can't be done. One thing you can do is install the wall covering with caulking to the studs and top/bottom plate and seal all the holes in it with foam or sealant.